(1) Field of the Invention
The present invention relates to a developing device for developing an electrostatic latent image on a photoreceptor and to a process unit and an image forming apparatus that include the developing device.
(2) Description of the Related Art
Electrophotographic image forming apparatuses, such as a printer, copying machine, etc. normally function as follows. A developing device forms a toner image corresponding to image data on a photoreceptor drum. The toner image is then transferred to a recording sheet, such as recording paper, an OHP sheet, etc., either directly or indirectly via a component such as an intermediate transfer belt. Finally, a fixing device fixes the toner image to the recording sheet.
The developing device may use either mono-component developer, which includes only toner, or two-component developer, which includes toner and magnetic carrier. A developing device that uses two-component developer is normally provided with a developing roller, a feed screw, and a mixing screw inside housing that contains the two-component developer. The developing roller develops the electrostatic latent image on the photoreceptor drum. The mixing screw supplies the feed screw with two-component developer, and the feed screw feeds the developing roller with the two-component developer. The photoreceptor drum, feed screw, and mixing screw are parallel to each other in an approximately horizontal position.
The two-component developer in the housing is stirred and mixed during transport by the mixing screw. As a result, the toner becomes statically charged. Statically charged toner is transported along with the carrier to the feed screw. The feed screw transports the two-component developer in the opposite direction as the mixing screw. Toner and carrier transported by the feed screw are provided to the developing roller.
Carrier provided to the developing roller forms a magnetic brush on the peripheral surface of the developing roller. Toner that adheres to the magnetic brush develops the electrostatic latent image on the photoreceptor drum. Toner and carrier that are not provided to the developing roller are transported from a downstream edge of the feed screw to an upstream edge of the mixing screw, the terms downstream and upstream referring to the direction of transport of each of the screws. The mixing screw and the feed screw thus circulate toner and carrier through the inside of the housing.
In order to provide the photoreceptor drum with a constant amount of developer, it is preferable for the photoreceptor drum, developing roller, feed screw, and mixing screw in the image forming apparatus to be arranged in a horizontal position. When the image forming apparatus is not placed horizontally, the developer in the housing tends to accumulate in one location, causing the amount of developer provided to the photoreceptor drum not to be constant. The quality of the toner image formed on the photoreceptor drum may therefore degrade.
For example, if the developing device is inclined so that the upstream edge of the mixing screw is positioned lower than a downstream edge of the mixing screw (respectively corresponding to the downstream edge and an upstream edge of the feed screw), the developer in the housing flows to and accumulates at the upstream edge of the mixing screw (i.e. the downstream edge of the feed screw).
The mixing screw is provided in a mixing chamber, and when the developing device is thus inclined, the upper level of the developer in the mixing chamber rises higher than the transport range of the mixing screw at the upstream edge. The mixing screw therefore cannot stably transport developer that accumulates in this area to the upstream edge of the feed screw. Accordingly, the amount of developer transported to the feed screw decreases, which results in a decrease in the amount of developer fed to the developing roller. Toner thus cannot be stably fed to the photoreceptor drum.
In recent years, demand has increased for compact, lower-cost image forming apparatuses. The size of image forming apparatuses has been reduced by making developing devices smaller through a decreased amount of developer in the housing of the developing device and a reduction in size of the mixing screw and other components. Even when the developing device is positioned horizontally, i.e. not inclined, the amount of developer transported by the feed screw in such a compact image forming apparatus is decreased.
Accordingly, as described above, if the upstream edge of the mixing screw is positioned lower than the downstream edge, then even a slight inclination angle has a higher probability of impeding transport by the feed screw of a predetermined amount of developer. If the predetermined amount of developer is not transported to the developing roller, excellent development of the electrostatic latent image on the photoreceptor drum may not be possible.
Patent Literature 1 (Japanese Patent Application Publication No. 2001-117337) discloses providing a rectifying fin on the peripheral surface of a developing sleeve to rectify developer that flows in the direction in which the developing device is inclined. The rectifying fin redirects the developer towards the central region of the direction of transport of the developing sleeve.
Furthermore, Patent Literature 2 (Japanese Patent Application Publication No. 2002-278270) discloses providing a transport screw, fins, and a balancer. The transport screw transports, in the direction of the axis of the developing roller, developer that is removed from the developing roller by a doctor blade. The fins are provided in a separator that causes the transport screw to transport developer downstream. The fins are at an incline and guide developer to an upstream edge of the transport screw. Depending on where the developer accumulates, the balance changes the inclination of the fins.
The structures disclosed in Patent Literature 1 and 2 make the amount of developer transported from the feed screw to the peripheral surface of the developing sleeve uniform when the developing sleeve is inclined in a predetermined position. However, neither of the structures in Patent Literature 1 and 2 overcomes the problem of decreased transport efficiency by the mixing screw when the upstream edge of the mixing screw is lower than the downstream edge.